1. Field of the Invention
This invention relates to apparatus for chemical vapor deposition (CVD) of coatings onto substrates such as flat glass substrates, and, more particularly, to apparatus suitable for forming metal-containing and like films having advantageous properties in an efficient and economical manner.
2. Description of the Prior Art
The desirability of applying uniform coatings to a flat glass substrate for the purpose of varying its thermal, optical and/or electrical properties has long been recognized. Such coatings generally consist of a metal or metal oxide, particularly tin oxide. In such process, a hot, freshly formed glass ribbon travels from a flat glass forming section to an annealing section, where the coating is deposited on one face thereof. The coating applicator generally consists of one or more nozzles which direct a spray of coating chemical reactant in a carrier gas onto the exposed face of the glass ribbon. Then reaction by-products and unused coating compound are removed by anexhaust duct.
Conventional coating apparatus, systems and processes are disclosed in such U.S. Pat. Nos. as 3,674,453; 3,681,042; 3,689,304; 3,841,858; 3,850,679; 3,852,098; 3,876,410; 3,888,649; 3,964,432; 3,974,432; 4,123,244; 4,125,391; 4,130,673; 4,144,362; 4,188,199; 4,206,252; 4,293,173; 4,293,326; 4,294,594; 4,325,987; 4,329,379; 4,330,318; 4,349,372; 4,387,134; 4,397,259; 4,469,045; 4,476,158; 4,500,567; 4,536,204; 4,584,206; 4,585,674; 4,597,984; 4,598,023; and 4,601,917; in EPO No. 0188962; an article entitled "Production of Chemical Vapor Deposition Coatings" by Nicholas M. Gralenshi, Watkins-Johnson Co., Scotts Valley, California 95066, pages 102-144; and an article entitled "Coating of Glass by Chemical Vapor Deposition" by John M. Blocher, Jr. in Thin Solid Films, No. 77, pages 51-63 (1981); the entire disclosures of which are incorporated by reference herein.
In the CVD process, it is desired to apply coatings to flat glass substrates which are (1) uniform, (2) haze-free, (3) have a low electrical resistivity; and which are formed during (4) a relatively short deposition time.
Although each of these requirements can be met individually, it is exceedingly difficult to satisfy all of them at the same time. For example, high substrate temperatures will provide short deposition times. With coating chemicals containing a dopant precursor, high substrate temperatures result in films having lower electrical resistance. Moreover, high deposition temperatures favor increased haze in the coatings formed. Similarly, haze-free films can be achieved using a low surface temperature, and relatively low water vapor content in the coating gases, albeit at the expense of reduced deposition rate and poorer electrical properties. Uniform coatings also require the application of a substantially uniform coating chemical composition at uniform vapor velocities on each spot on the surface of the glass substrate, which is difficult to achieve with known nozzle applications. In short, conventional CVD systems are found to be deficient with respect to one or more of these film qualities and process parameters.
Furthermore, with prior art systems, a larger exhaust for removing spent gases is generally required. Such large exhausts tend to draw outside gases, e.g. air, into the coating zone, which dilutes the concentration of coating vapors. On the other hand, a small exhaust system will allow coating gases to escape from the coating zone and become admixed with the outside atmosphere. In both cases, the diluted vapors which contact the glass substrate produce unacceptably hazy films. Moreover, large amounts of exhaust reduce the efficiency of use of coating material, and increase the cost of recovery of coating material from the exhaust gases.
Many of these prior CVD systems use a nozzle or nozzles which are positioned adjacent to the surface of the substrate, and which are provided with clearances between the nozzle and substrate and between the exhaust port and substrate. These open systems result in outside air being attracted to and mixed with coating vapors causing haze, for example, as shown in U.S. Pat. No. 4,123,244.
Further, with conventional CVD systems, the assumed relationships between coating gas velocity, chemical concentration and chemical consumption, with nozzle slit width, have provided less than satisfactory coating applicators. Specifically, such relationships have required a nozzle applicator with a rather narrow nozzle slit width, which causes extreme problems with film uniformity and nozzle clogging. In addition, such systems consume a large amount of chemical reactant, resulting in an uneconomical process. On the other hand, systems which use a low gas velocity or low chemical concentration tend to effect deposition at a rate which is controlled primarily by diffusion of chemicals to and from the substrate surface, which can produce a coating which has a rough surface and hazy characteristic.
Accordingly, it is an object of the present invention to provide an improved CVD process, apparatus and system for depositing advantageous films on a flat glass substrate.
Another object of this invention is to deposit a uniform, haze-free, low electrical resistivity film on a flat glass substrate at a high deposition rate by a CVD process.
Still another object herein is to provide a coating applicator which applies a high concentration of coating chemical at relatively high jet speeds to a flat glass substrate.
It is a further object of the present invention to provide a process of coating a flat glass substrate by CVD with little cooling of the substrate.
It is still another object of this invention to provide a process and apparatus for coating a flat glass substrate with little or no intermixing of the vaporized chemical and carrier gas with outside air.
A feature of the invention is the provision of an improved CVD process, apparatus and system for chemical vapor deposition of a visibly reflective or infrared reflective film onto a moving float glass ribbon under reaction rate controlled conditions with substantially no intermixing of coating chemical vapors with the outside atmosphere.
These and other objects, features and advantages of the present invention will become readily apparent from the following detailed description thereof which is to be read in connection with the accompanying drawings, in which: